Commitments and Contingencies - Schedule of Long-term Commitments (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Transportation Payments Due, 2019	$ 194
Future Minimum Transportation Payments Due, 2020	175
Future Minimum Transportation Payments Due, 2021	141
Future Minimum Transportation Payments Due, 2022	133
Future Minimum Transportation Payments Due, 2023	108
Future Minimum Transportation Payments Due, Thereafter	1,013
Total future minimum transportation payments due	1,764
Future Minimum Capital Projects Payments Due, 2019	298
Future Minimum Capital Projects Payments Due, 2020	89
Future Minimum Capital Projects Payments Due, 2021	33
Future Minimum Capital Projects Payments Due, 2022	8
Future Minimum Capital Projects Payments Due, 2023	2
Future Minimum Capital Projects Payments Due, Thereafter	6
Total future minimum capital projects payments due	436
Future Minimum Fuel and Gas Supply Payments Due, 2019	257
Future Minimum Fuel and Gas Supply Payments Due, 2020	106
Future Minimum Fuel and Gas Supply Payments Due, 2021	3
Future Minimum Fuel and Gas Supply Payments Due, 2022	3
Future Minimum Fuel and Gas Supply Payments Due, 2023	1
Future Minimum Fuel and Gas Supply Payments Due, Thereafter	0
Total future minimum fuel and gas supply payments due	370
Future Minimum Long-term Service Agreements Payments Due, 2019	7
Future Minimum Long-term Service Agreements Payments Due, 2020	6
Future Minimum Long-term Service Agreements Payments Due, 2021	6
Future Minimum Long-term Service Agreements Payments Due, 2022	7
Future Minimum Long-term Service Agreements Payments Due, 2023	11
Future Minimum Long-term Service Agreements Payments Due, Thereafter	78
Total future minimum long-term service agreements payments due	115
Future Minimum Operating Leases Payments Due, 2019	2
Future Minimum Operating Leases Payments Due, 2020	2
Future Minimum Operating Leases Payments Due, 2021	2
Future Minimum Operating Leases Payments Due, 2022	2
Future Minimum Operating Leases Payments Due, 2023	2
Future Minimum Operating Leases Payments Due, Thereafter	34
Total future minimum operating leases payments due	44
Future Minimum Demand Side Management Payments Due, 2019	5
Future Minimum Demand Side Management Payments Due, 2020	1
Future Minimum Demand Side Management Payments Due, 2021	0
Future Minimum Demand Side Management Payments Due, 2022	0
Future Minimum Demand Side Management Payments Due, 2023	0
Future Minimum Demand Side Management Payments Due, Thereafter	0
Total future minimum demand side management payments due	6
Future Minimum Payments Due, 2019	763
Future Minimum Payments Due, 2020	379
Future Minimum Payments Due, 2021	185
Future Minimum Payments Due, 2022	153
Future Minimum Payments Due, 2023	124
Future Minimum Payments Due, Thereafter	1,131
Total future minimum payments	$ 2,735